Wednesday, November 18, 2015

A timer is a specialized type of clock for measuring time intervals. By function, timers can be categorized to two main types. Those are Counts upwards and counts downward.

Timers originally designed to fulfill a need in industry for a means of keeping time on certain devices. Originally, these timers were mechanical devices and used clockwork mechanisms as a means of keeping a regular time. The invention of two electromechanical timer designs allowed for more precise time measurement. The first uses the principle of heat expansion to increase the temperature of a metal finger made of two different metals with differing rates of thermal expansion. As electric current flows through the metal, it begins to heat and one side expands more quickly than the other does, which in turn, moves the electrical contact away from an electrical switch contact. The second uses a small AC motor, which turns at a predetermined rate due to the application of an alternating current.

Finally, digital timers invented. Digital logic circuits are now so cheap that it has become a better investment to buy a digital timer than a mechanical or electromechanical timer. Individual timers implemented with single chip circuits.

This is a very simple adjustable digital timer circuit based on the PIC16F628A microcontroller and it can be programmed to schedule the on and off operation of an electrical appliance. This timer consists of three parts: power supply, control circuit and display. Working voltage of the circuit is 5v - 12v. It depends on the relay voltage. If you use 5v relay then you can omitted the LM7805 regulator IC and apply 5v directly. Otherwise, you have to use regulator IC and apply suitable voltage according to the relay voltage. The schematic is very simple and accurate of the circuit is very good. PIC use its internal oscillator.

Programmable Digital Timer Schematic

There are two versions of hex file are available. Those are "4dg_tmr_min.hex" and "4dg_tmr_hr.hex". The first file for the minute timer. It display minutes and seconds. Adjustable time is 1 second to 60 minutes. Other hex file for hourly timer and its adjustable time is 1 minute to 24 hours. This will display hours and minutes on the seven segments.

Configuration

If the time runs too fast or too slow, you can able to adjust the speed by changing the value of Eeprom address 0. Default value is 44 (0x2c). Typical value is 59 (0x3B). Maximum is 255 (0xFF). In repeat mode delay time before restart the timer, determine by value of Eeprom address 3. Default value is 10 (0x0A). Maximum is 255 (0xFF). See below picture for more details.

Eeprom Configuration

Operation of the timer

This circuit uses 5 push buttons to control the their functions.

START/PAUSE: When the timer is on, the device is in pause condition even the switch was in closed position. Pressing this button, you can switch between the start and pause timer.

FOR/BACKWARD: This allows you to select counter mode. Either upwards or count down.

REPEAT: When the timer reaches 00:00, it starts again from previous value you set.

LEFT/RIGHT: This allows you to change values on display. The selected digit is incremented by pressing those buttons and values on the display are stored to the Eeprom.

Now connect device you want to operate, through the relay. Set the desired time using left and right buttons and press start. When the timer reaches 00:00, relay will activate.

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